The invention relates to a blocking oscillator for use in a buzzer, and more particularly, to a transistor oscillator circuit which is stabilized against changes in ambient temperature and fluctuations of a supply voltage.
A blocking oscillator for use in a buzzer and comprising npn or pnp transistor is well known, and is described in German Laid-open Application No. 2,110,918 and U.S. Pat. No. 3,564,542, for example. One form of known blocking oscillator comprising an npn transistor is illustrated in FIG. 1 where terminals 1 and 2 are adapted to be connected with a power supply. A current flow through resistor and control coil L.sub.2 establishes a bias voltage across the base and emitter of a transistor T, which therefore conducts to permit a current flow through its collector-emitter path including a drive coil L.sub.1. The both coils L.sub.1 and L.sub.2 are disposed on a common core to provide an inductive coupling between them. The current flow through the drive coil L.sub.1 induces a voltage across the control coil L.sub.2 which is of a polarity to reduce the base-emitter voltage V.sub.BE of the transistor T below the bias voltage, thus rendering the transistor cut-off. Thereupon a reverse e.m.f. is induced across the control coil L.sub.2 to maintain the transistor cut-off. As the reverse e.m.f. is discharged through a loop including the emitter and base of the transistor and the voltage V.sub.BE across the base and emitter path of the transistor increases, it is again rendered conductive. The process is periodically repeated at a rapid rate, which determines the frequency with which an armature is magnetically driven by the core.
One of the requirements for a proper operation of the blocking oscillator is that the transistor T must be rendered conductive when the power is initially turned on. Another requirement relates to the voltage induced across the control coil L.sub.2 which must be sufficient to reduce the voltage across the base and emitter of the transistor below the bias voltage so that the latter is made cut-off. This requires that the transistor maintains a proper value of the bias voltage. However, when the oscillator is used in an environment having a varying ambient temperature, a variation in the operational characteristic of the transistor prevents a proper operation from being achieved if the bias voltage across the base and emitter is maintained constant.
Reference is made to FIG. 4 which graphically shows the collector current I.sub.C plotted against the base-emitter voltage V.sub.BE of the transistor at ambient temperatures of +80.degree. C, +25.degree. C and -25.degree. C. Assuming that a proper oscillator operation is achieved at an ambient temperature of +25.degree. C when the operating point of the transistor T is chosen at a on the graph, a change of the ambient temperature to 80.degree. C will shift the operating point of the transistor to b. Once it conducts, the transistor cannot be turned off and remains on since the induced voltage across the control coil is insufficient to reduce the voltage V.sub.BE below the threshold voltage of the transistor. On the other hand, when the ambient temperature changes to -25.degree. C, the voltage V.sub.BE is less than the threshold voltage, failing to turn on the transistor.
U.S. Pat. No. 3,887,914 issued to the common assignee as the present assignee proposed a circuit arrangement as shown in FIG. 2 of this application in which a diode D is connected in series with the control coil L.sub.2 in order to compensate for the effect of a change in the ambient temperature on the base-emitter voltage V.sub.BE. In this instance, the diode comprises the same type of semiconductor material as the transistor, and thus is formed of silicon where the transistor is formed of silicon, and is preferably placed under the same thermal condition as the transistor. This achieves a forward voltage (V.sub.F) versus forward current (I.sub.F) characteristic of the diode which is approximately similar in configuration to the V.sub.BE - I.sub.C characteristic of the transistor. A variation in the characteristic due to temperature changes will also become similar in the both devices (see FIGS. 4 and 5). In this manner, the bias voltage can be varied so as to compensate for a change in the threshold voltage of the transistor.
With continued reference to FIG. 4, we will consider a change of the ambient temperature from +25.degree. C to +80.degree. C. The operation of the transistor will then shift to the leftmost characteristic curve, and if the transistor was properly operating at point a under the ambient temperature of +25.degree. C, the shift of the diodes' characteristic (see FIG. 5) as a result of the temperature change will reduce the bias voltage, bringing the operating point of the transistor to point a'. It will thus be seen that the transistor now operates in the same way as it operated under the ambient temperature of +25.degree. C. Thus it is possible to maintain an oscillation of the transistor for all temperature changes, whereby it is assured that the drive coil L.sub.1 will induce a periodically varying electromagnetic field in the core 3.
However, the improved blocking oscillator suffers from the dependency on the supply voltage in that while the V.sub.BE -I.sub.C characteristic of the transistor remains unchanged (assuming no temperature change), the bias voltage of the transistor will vary with a fluctuation of the supply voltage. FIG. 3 illustrates a conventional way of suppressing a fluctuation of the supply voltage by connecting a Zener diode ZD across the oscillator circuit and connecting the parallel combination across the power supply in series with a resistor R.sub.2. However, the connection of the resistor R.sub.2 requires an increased supply voltage and also represents a power loss.